The present invention relates to a hydraulic control system in a working machine.
There exists a working machine, such as a hydraulic shovel, that includes a plurality of hydraulic actuators to which pressurized oil is supplied from a hydraulic pump. A conventional hydraulic circuit of the working machine is configured such that oil discharged from the hydraulic actuators is returned to an oil tank. In the hydraulic shovel, for example, oil discharged from a head-side oil chamber of a boom cylinder is returned to the oil tank when the boom cylinder is retracted to lower a working portion. The oil in the head-side oil chamber of the boom cylinder, which holds a weight of a front working portion, contains high pressure and hydraulic energy. However, the high hydraulic energy is returned to the oil tank without being used further, with a resultant loss of energy.
The hydraulic energy contained in the discharged oil from the hydraulic actuator is pressure-accumulated in an accumulator and pressure-accumulated oil in the accumulator is allowed to merge into a discharge passage of a hydraulic pump in order to recover and recycle the hydraulic energy of the discharged oil from the hydraulic actuator (see WO 98/13603, for example). Further, the pressure-accumulated oil in the accumulator is allowed to merge into the pump discharge passage with a pressure of the pressure-accumulated oil being unchanged or being increased by a pump motor in accordance with a pressure difference between an accumulated pressure in the accumulator and a discharged pressure from the pump.